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or. , I ','I. ",'\. . ., . M[\u8<J . '. .. SUMMARY DESCRIPTION AND REVIEW OF EXISTING GUNNISON BASIN MODEL lNTRODUcnON ll1is document is a summary description and critical review of the eXJsling basm model. It is based on the executive summary and excerpts from Task Memorandum No. , of the Colorado Water Resources and power Development Authority's Phase 1 Study, on public comments received in the scap. iog of the current planning model development project, and 'on Hydrosphere's own technical review of the model. A diagram of the model network and tables listing important data and information support- ing the summary are anached at the back ofthis document. MODEUNG APP.ROAQI The basin model was developed using a network modeling system and runs on IBM-compa(ible microcomputers. in the model, the basin is characterized as a network of links represeming inflows, sneam reaches, diversions, reservoirs, and demands. The priorities placed on various links In the network are derived from water nghts considerations. These priorities control the allocation of water to various uses represemed in the model. Operating rules for various facilities are implemented through special computer code which adjusts the priorities assigned to specific links, such as those which represent reservoir storage or relc;l.Ses. The model operates on a monthly time step over a 1952-83 study period. In each month, [he model solves for an opumal allocation of available water supplies according to the priorities placed on various links by the user. Reservoir carryover storage and lagged return nows are the connections be- tween one month's solurion and the next. A diagram of the model network is attached to the back of this docUI:nenl. STIJDY PERIOD IIYDROLOGY lnDows Files of model inflows at 59 sites were derived by U.S. Bureau of Reclamation (USBR) staff in Grand Junction. These inflows are of threc. typcs, 1) vjrgin flows, 2) gaged flows, and 3) imputed ,each gains. Virgin flows at gaged sires were derived from historical flow records corrected for upstream reser- voir operations. basin imports and exportS, and irrigation depletions. Historical irrigation depletions were estimated from consumptive use rates derived from the CRSS database, a.crea.ges obtained from the USBR. the SCS, and the Gunnison CounlY assessor, and from detailed operaling studies of selected ditches. Virgin flows at ungaged siles were developed [rom virgm flows at "reference gages" corn=-crcd for area and elev~llion differences; Table 1 li':;,lS lhcse reference gages. Gag~d OOVolS Wt.:TC lIsed 1O rcprcst:11I inflows from the North and SmHh Forks of the Gunnison and the inllows to R.id~way Reservoir. !teach ~ains imputed from gaged flows were used lo rcprcscnl ac- cretions along the Uncompahgre I{JVtr above the SOLlth Canal and along the Gunnison below Della.